I use the square gates generally, mostly because having the diagonals easy to hit cleans up my execution in fighters.
Octagonal gates don't do anything for me. I used to sac them to make circular gates before they were readily available. Sold about 70 or 80 of them on shoryuken before Toodles started machining them.
The circle gate are fun for tooling around on since they are so smooth. I don't find them to be nearly as precise as a JLF with a square gate, but I'd like to try a circle gate with Toodles' Spark CE optical switch assembly. Supposedly you can adjust a pot that makes it more or less sensitive, which could make the diagonals more responsive.